Red blood cell transfusions are routinely given to critically ill patients. Recent epidemiological studies have shown that red cell transfusions are associated with the development of lung injury, increased morbidity and mortality in the critically ill. The etiology underlying this association remains uncertain, as a clinical benefit has not been convincingly demonstrated with leukoreduction. The main premise of this study is that transfusion of the non-leukocyte component of red cell concentrates can pre-dispose the recipient to acute lung inflammation and injury during sepsis. We have developed an in vivo murine model whereby purified erythrocyte transfusion promotes neutrophilic accumulation in the lungs. The main goal of this proposal is to determine whether transfusion of purified erythrocytes during septic inflammation can lead to increased oxidative stress through the generation of reactive oxygen species (ROS), endothelial cell activation and lung injury. In Specific Aim 1 we will determine whether the non-leukocyte component of red cell concentrates can promote neutrophil mediated lung injury in a mouse model of sepsis. In Specific Aim 2, we will determine the impact of erythrocyte concentrates on endothelial cell activation, neutrophil-endothelial adhesion, and trans- endothelial migration in vitro. In Specific Aim 3, we will examine the effects of erythrocyte transfusions on the generation of reactive oxygen species ROS both in vitro and in our in vivo model using gp47 null mice deficient in NADPH oxidase activity. Knowledge derived from these studies may elucidate one mechanism underlying the consequences of red cell transfusion and provide new insight to a common problem in the critically ill.